This invention relates to the suppression of transients which can occur in conventional optical wavelength division multiplex (WDM) communication systems.
1. Field of the Invention
It has been shown in J. L. Zyskind et al, xe2x80x9cFast power transients in optically amplified multi-wavelength optical networksxe2x80x9d, Optical Fibre Communication conference, vol. 2, OSA Technical Digest Series, PD 31-1, San Jose, 1996, that power transients in chains of highly pumped, deeply saturated erbium doped fibre amplifiers were demonstrated to occur on time scales much faster than those for an individual amplifier. Such fast power transients constitute a major issue for optical networks in which channels are added and dropped either due to network reconfiguration or failures. The surviving channels will suffer error bursts if their powers exceed the dynamic range of the terminal receiver. Protection against such error bursts must be fast enough to limit the surviving channel power excursions.
2. Description of the Related Art
According to a first aspect of the present invention, an optical level controller for a wavelength division multiplexed optical communication network comprises one or more variable gain elements adapted to suppress transient amplitude fluctuations in traffic signals caused by the adding or the dropping of traffic signals at one or more points in the optical communication network, wherein the variable gain element is controlled through a negative feedback loop adapted such that a response time of the negative feedback loop is shorter than the time scale of the transient amplitude fluctuations.
In the present invention, a high speed variable gain element is provided to protect a network from fast power variations.
In one preferred embodiment, the variable gain element is a semiconductor laser amplifier in which the bias of the amplifier is controlled by the negative feed back loop.
In another preferred embodiment, the variable gain element is a Mach-Zehnder optical modulator in which the bias of the modulator is controlled by the negative feedback loop.
Preferably, the negative feedback loop comprises an optical coupler to split a portion of the output of the respective variable gain element onto the negative feedback loop, and a photodiode to detect the split portion of the variable gain element.
Preferably, the negative feedback loop comprises a low pass filter to provide a slowly varying dc voltage for control of the respective variable gain element.
According to a second aspect of the present invention, a wavelength division multiplexed optical communication network comprises a voltage level controller in accordance with the first aspect of the present invention.
The voltage level controller may be provided to condition signals entering a receiving station of the network or alternatively located at an optical cross-connect of the network.